This invention discloses processes and apparatus for the continuous polymerization of an elastomer monomer into an elastomeric polymer, and reaction of said resulting polymerized elastomer with a monovinylaromatic monomer to form an elastomer-modified transparent monvinylaromatic material.
Processes and apparatus for the continuous production of polymerized elastomers such as polybutadiene, polyisoprene, and their copolymers, such as styrene-butadiene rubber (SBR) and styrene-butadiene-styrene (SBS) rubber are well known. Likewise processes and apparatus for producing monovinylaromatic compounds such as polystyrene, alphamethyl-styrene and others have long been known. Also well known are processes and apparatus for making elastomer-modified monovinylaromatic compounds such as polybutadiene-modified polystyrene to serve as high-impact materials. One certain class of such materials is denoted as High Impact Polystyrene (HIPS) which is made from polybutadiene rubber and styrene monomer. Another class of materials which are made from various copolymers and styrene monomer is classified as Transparent Impact PolyStyrene, or TIPS.
The primary difference between a HIPS material and a TIPS material arises from the difference in the morphologies of the two materials"" rubber phases. In the HIPS material, the rubber is present as a distribution of different sized, well-defined spherical particles, ranging from about 0.5 up to about 15 microns in diameter. In TIPS materials the rubber phase is present in xe2x80x9cdomainsxe2x80x9d that have dimensions that will not refract visible light, and therefore appear to the human eye as transparent.
TIPS materials can be manufactured by extruding blends of an SBS (styrene-butadiene-styrene) rubber that has been blended with a general purpose polystyrene (GPPS). For example, blends containing 40% by weight of a commercial SBS rubber sold by TotalFinaElf of Paris, France, under the tradename xe2x80x9cFinaclear 520xe2x80x9d, when blended with 60% by weight of TotalFinaElf""s commercial GPPS material sold under the tradename xe2x80x9cFina GPPSxe2x80x9d, produce a TIPS that can be used for making clear delicatessen food containers and transparent beverage cups.
However, blending of rubber and polystyrene materials to form TIPS suffers from a major disadvantage, i.e., the cost of manufacture. The many steps of manufacturing a separate commercial rubber material and a separate commercial polystyrene, then having to granulate and then blend the two materials and extrude them, then further pelletize the extruded TIPS materials, introduces numerous expensive steps into the process. An even further disadvantage of such blended TIPS materials involves the great likelihood that the final product will not be totally clear but, due to poor blending and dispersion techniques, will also suffer from the presence of xe2x80x9chazexe2x80x9d in the material that greatly reduces its attractiveness as containers for food and beverage. The commercial industry has long needed and lacked a process for manufacturing TIPS materials directly in a reactor to provide ultraclear reactor-grade materials having no haze due to the elimination of the dispersion problem in blending operations. Also, the direct reaction of rubber in polystyrene would be much cheaper than the blending method of manufacture.
Another conventional method of making HIPS and TIPS materials involves starting with a polybutadiene rubber or an SB (styrene-butadiene) copolymer, which is usually supplied in large bales which have to be ground into small particles and then added to a diluent or solvent such as normal hexane, cyclohexane or ethylbenzene, and mixing this with a styrene monomer. This mixture is then polymerized in a typical HIPS or TIPS reactor system, usually consisting of at least one Continuous Stirred Tank Reactor (CSTR). Some examples of conventional rubber-modified polystyrene manufacture are disclosed in U.S. Pat. Nos. 4,271,060; 4,375,524; 4,495,028; 4,567,232; and, 4,686,086. In U.S. Pat. No. 4,777,210 to Sosa et al, a HIPS process is disclosed which utilizes a first CSTR type of reactor denoted as a Pre-Inversion Reactor, or PIR, to react the mixture of styrene monomer and dissolved polybutadiene rubber up to a solids level just below the inversion point. The written descriptions of the above-cited six U.S. patents are hereby incorporated herein in by reference.
Two patents to Knoll, U.S. Pat. No. 5,795,938 and 5,990,236, teach a process for making rubber modified styrenics in which a first reactor polymerizes a pure styrene monomer in cyclohexane to 99.99% conversion to polystyrene; and in a second reactor a 60/40 mixture of butadiene and styrene monomer in cyclohexane solvent is reacted to about 15% solids using a butyl lithium accelerator. The resultant butadiene/styrene solution is then physically mixed with the 99.99% polystyrene material to form a blended rubber-polystyrene material.
An improvement over the above cited conventional processes is that disclosed in U.S. Pat. No. 6,143,833 to Klussman et al, wherein a polybutadiene rubber is manufactured using butadiene monomer dissolved in a solvent, which rubber is polymerized in a reactor system in the same physical area as a HIPS reactor system. This material is then run through a solvent exchange system to remove the solvent from the butadiene rubber and then the rubber is added to the HIPS reactor system with styrene monomer to complete the HIPS process. This process removes the steps of making a rubber at a conventional rubber manufacturing facility, drying and compressing the rubber, transporting it to the HIPS plant, grinding the rubber bales into granules, dissolving the granules in a diluent compatible with styrene polymerization, and then physically mixing the dissolved rubber with the styrene monomer and polymerizing the final product in the HIPS reactor system. Usually in such conventional systems, the rubber is manufactured at a completely different location altogether, and sometimes even in a different country, from where the final HIPS products are made.
Another improvement of the prior art processes of HIPS manufacture involves the processes and apparatus disclosed in a copending application Ser. No. 09/823,535, filed by Sosa et al, on Mar. 30, 2001, entitled xe2x80x9cPROCESS FOR MANUFACTURING IMPACT RESISTANT MONOVINYLAROMATIC POLYMERSxe2x80x9d. This process utilizes as a first reactor a stirred elongated upflow reactor to replace the initial preheater and CSTR type of reactor of conventional HIPS processes to provide higher conversion rates, better temperature control and better rubber particle size distributions. The written descriptions in the above described Klussman, et al, patent and Sosa, et al, patent application are incorporated by reference herein.
All of the prior art processes and apparatus described hereinabove except the Klussman et al patent, suffer from the disadvantage of the many steps required to make HIPS material, including manufacturing rubber prior to incorporating it into the styrene polymerization process, transporting the rubber to the HIPS site, and then grinding and dissolving the rubber in a styrene-compatible solvent and incorporating it into the HIPS reactors along with the styrene monomer.
The present invention overcomes the disadvantages of the prior art for manufacturing transparent impact polystyrene by providing processes and apparatus for polymerizing an elastomer which possesses a random SB-block S structure, using a solvent compatible with styrene polymerization, in an apparatus immediately adjacent to the styrene reactor system; then feeding the elastomer/solvent solution into a solvent exchange system where solvent is exchanged with styrene monomer, then feeding the elastomer and styrene monomer into the impact polystyrene reactor system to polymerize the styrene monomer and form the transparent impact-resistant polystyrene.